For example, an ultrasonic linear actuator has a structure schematically illustrated in FIG. 20, wherein it is adapted to transmit extending and contracting of a piezoelectric element to a rod (driving shaft), and move a driven member (movable body) engaged with the rod with a predetermined frictional force, based on a speed difference between during extending and during contracting of the piezoelectric element. For example, when the rod is extended slowly, the driven member frictionally engaged with the rod is moved together therewith, as illustrated in FIGS. 20(A) and 20(B), and then when the rod is contracted instantaneously enough to exceed the predetermined frictional force, the driven member is left at a extend position due to inertia, as illustrated in FIGS. 20(B) and 20(C). Based on repeatedly performing this motion, the ultrasonic linear actuator is operable to move the driven member in an axial direction of the rod. The ultrasonic linear actuator is also adapted to perform the extending instantaneously and then perform the contracting slowly, thereby allowing a direction of movement of the driven member to be reversed with respect to that as described above.
This type of ultrasonic linear actuator is structurally simple as compared to conventional Lorentz force-type motors and others, and capable of directly driving a load without using a speed reducing mechanism. Thus, as an example of its application, the following Parent Document 1 proposes a drive device configured such that the rod is set up in a direction of an optical axis of a lens, and a holding member of a focusing lens is engaged with the rod, thereby realizing autofocusing. In order to allow the driven member to be frictionally engaged with the rod, a magnetic force may be used, as well as a pressing force based on a spring or the like.
However, the above ultrasonic linear actuator has problems of low speed performance and poor efficiency as compared to other types of ultrasonic motors similarly using a piezoelectric element, such as standing wave-type and traveling wave-type ultrasonic motors. The difference is because, in other types of ultrasonic motors, the piezoelectric element is driven in a resonance region. When the piezoelectric element is driven in the resonance region, it becomes possible to increase a displacement (stroke) even using a low-voltage signal, and efficiently improve the speed performance. On the other hand, in the above ultrasonic linear actuator, a frequency of a drive signal is approximately 0.7 times as high as a resonance frequency as described later, so that the displacement (stroke) is as small as several μm at a maximum. Further, in the case of utilizing resonance, input energy is mostly used for mechanical vibration, whereas, in the case of utilizing no resonance, the energy is used for charge and discharge of an electrical capacitor made with a dielectric and constituting the piezoelectric element, which leads to poor efficiency.
FIG. 21 illustrates a relationship between respective displacements of the piezoelectric element and the driven member over time, in the ultrasonic linear actuator. As mentioned above, the driven member is moved based on the speed difference between during stretching and during shrinking of the piezoelectric element. For this purpose, a pseudo-sawtooth drive signal as illustrated in FIG. 22 is given from a drive circuit to the piezoelectric element. Then, displacements corresponding to respective oblique line sections in a sawtooth waveform of the drive signal are added up as a total displacement amount of the driven member, as illustrated in FIG. 21.
In the following Patent Document 2, the applicant of this application previously disclosed that a sawtooth displacement can be obtained by appropriately selecting a drive frequency even if a rectangular-wave voltage as illustrated in FIG. 23 is given. Further, a theoretical background thereof is disclosed in the following Non-Patent Document 1. The theory may be summarized as follows. A basic of a sawtooth waveform as illustrated in FIG. 24(C) can be obtained by adding, to a sinusoidal signal having a fundamental frequency as illustrated in FIG. 24(B), a second harmonic sinusoidal signal as illustrated in FIG. 24(A). In other words, the sawtooth waveform includes, as components, a plurality of sinusoidal waves having different frequencies. In this case, as long as there are at least first-order and second-order components among them, the signal becomes a sufficient level to drive the ultrasonic linear actuator. The displacement y can be expressed as the following formula (1):y=−sin(ωt)−0.25·sin(2ωt)  (1)
As conditions for obtaining such a sawtooth waveform, in cases where the drive frequency is low, it is necessary to form a drive signal into the sawtooth waveform exactly. However, when the drive frequency is increased to a certain extent, it becomes possible to produce a sawtooth displacement by inputting, into the piezoelectric element, the aforementioned rectangular wave having a frequency which is approximately 0.7 times as high as the resonance frequency. In the Patent Document 2, this characteristic is utilized to allow the actuator to be driven by a rectangular-wave voltage which is easily implementable in a product.
FIG. 25 illustrates a change in a movement speed of the driven member when a duty ratio and a frequency of the rectangular wave are changed. This graph is illustrated as FIG. 20 in the Non-Patent Document 1, wherein the resonance frequency of the piezoelectric element and the rod and the drive voltage of the piezoelectric element are set to 200 kHz and 6 Vp-p, respectively, and a frictional force of the driven member with respect to the rod is set to 300 mN. As is clear from FIG. 25, when the frequency of the rectangular wave is approximately 0.7 times as high as the resonance frequency, respective phases and gains of a sinusoidal wave as a first-order component and a sinusoidal wave as a second-order component each included in the rectangular wave have an adequate relationship, so that a sawtooth displacement is obtained, and the highest speed is obtained.
However, if the drive frequency is set to become coincident with the resonance frequency as in other types of ultrasonic motors, only the gain of the sinusoidal wave as a first-order component wave included in the rectangular wave is amplified as illustrated in FIG. 26(B), and, in contrast, the gain of the sinusoidal wave as a second-order component is reduced, so that an adequate sawtooth displacement as illustrated in FIG. 26(A) is not obtained. This means that it is impossible to drive the above ultrasonic linear actuator.
The ultrasonic linear actuator is structurally simple, and capable of directly driving a load without using a speed reducing mechanism, as mentioned above. Thus, based on higher-power characteristics (improvement in movement speed and energy efficiency), it can be expected to develop application to new products. For example, it can be expected to use the actuator for a new purpose such as artificial muscle of a humanoid robot. Further, it is possible to promote microminiaturization using a piezoelectric thin film, specifically, structurally simplify a Langevin vibrator using a thin film or a non-laminate structure. Thus, it can also be expected to develop application to a micromachine such as an actuator for use in a distal end of an endoscope. On the other hand, in existing products, for example, in image stabilization of a camera, based on the higher-power characteristics, it becomes possible to displaceably drive a larger image sensor at a high speed, and suppress power consumption and heat generation even in a continuous drive mode such as a video recording mode.